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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../container/array/operator_at.html below:

std::array<T,N>::operator[] - cppreference.com

reference operator[]( size_type pos );

const_reference operator[]( size_type pos ) const;

Returns a reference to the element at specified location pos.

If pos < size() is false, the behavior is undefined.

If pos < size() is false:

• If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
• If the implementation is not hardened, the behavior is undefined.

Reference to the requested element.

Constant.

Unlike std::map::operator[], this operator never inserts a new element into the container. Accessing a nonexistent element through this operator is undefined behavior, unless the implementation is hardened(since C++26).

The following code uses operator[] to read from and write to a std::array<int, N>:

#include <array>
#include <iostream>
 
int main()
{
    std::array<int, 4> numbers{2, 4, 6, 8};
 
    std::cout << "Second element: " << numbers[1] << '\n';
 
    numbers[0] = 5;
 
    std::cout << "All numbers:";
    for (auto i : numbers)
        std::cout << ' ' << i;
    std::cout << '\n';
}

Output:

Second element: 4
All numbers: 5 4 6 8

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